GFCI systems have been improved to provide prompt detection and isolation of system faults while minimizing nuisance interruptions. See for example U.S. Pat. No. 7,301,739, which is assigned to the present assignee and which is herein incorporated by reference in its entirety. U.S. Pat. No. 7,301,739 also describes a means to compensate for small capacitive currents that normally flow throughout a three-phase power system during low- or high-level ground-faults, thereby further minimizing tripping of the non-faulted circuits. However, GFCI systems in most common use at this time have been designed with the primary objective of permitting reliable, high-speed ground-fault interruption for single-phase power systems operating at relatively low voltages, e.g., typically less than 125 volts phase-to-ground.
Although conventional GFCI systems can be reliable for three-phase power systems rated 480 volts phase-to-phase (i.e., 277 volts phase-to-neutral or ground) and above, “nuisance” tripping may become more prevalent if used with polyphase systems operating above 125 volts phase-to-ground due to significant capacitive charging currents that are characteristic of insulated phase conductors associated with feeder or branch circuits. These charging currents result from the distributed capacitance of insulated phase conductors in close proximity to grounded surfaces or conductors. For example, in 277 V phase-to-ground, three-phase, multiple feeder systems having one phase faulted to ground, the magnitude of the capacitive charging currents on the unfaulted phases of the non-affected feeders can easily reach a magnitude that will “false trip” the non-affected feeders' GFCIs.
As such, a need exists for a GFCI system that can provide improved tripping “intelligence” or “security” for polyphase power systems at all system voltage levels, and in particular, polyphase power systems operating above 125V phase-to-ground.
A further need exists for an improved GFCI system for three-phase, multi-feeder systems having a ground-fault sensor (GFS) sensitivity corresponding to the lower limit of the human “let-go” threshold of current, i.e., 4-6 mA, which will reliably trip on the affected feeder(s) without causing false interruptions on the unaffected feeder(s). GFSs designed to trip at such low currents typically include current transformers that may be susceptible to magnetic saturation due to high-level ground current conditions on involved branch or feeder circuits. Magnetic saturation of the transformers may cause the corresponding GCFIs to falsely trip non-faulted circuits.